A photoplethysmograph is a device used for the measurement of a peripheral pulsatile blood flow. It senses blood flow by means of a probe placed on the surface of the skin of any part of the body. The probe contains a tiny light source and a specially selected photo-sensitive cell that responds to light absorbed by the arterial blood in the peripheral vascular bed over which the sensor is placed. Since the cell responds to the light absorbed by the blood in its view, the amount of pulsating light it registers is proportional to the amount of pulsating arterial blood in its field.
In practice, however, the amplitude of electrical signal produced by the photo-sensitive cell not only varies with the amount of pulsation of blood, but also varies with the pressure of application of the probe to the skin surface. As the pressure is gradually increased, at first the amplitude of the signal gradually rises. It reaches a maximum, and then, with further increases in pressure, once again diminishes until it finally drops off to zero. The drop-off of signal beyond the maximum point as the pressure increases is easily explained in terms of the high pressure's squeezing the blood vessels closed and thus actually cutting off the blood flow. The increase in amplitude at first with increasing pressure is caused by changes in the amount of steady state static light returned to the photocell. It is the average amount of light returned to the photocell that establishes the average resistance of the cell (the operating point) and thus the average gain or sensitivity of the cell as regards the pulsatile signal which is being measured.
One of the problems involved in the photoplethysmographic pickup of the blood flow pulse is that variations in the amount of scattered light reaching the photocell cause variations in the operating point of the photocell. This adversely affects the accuracy of the measurement.
Another problem associated with the photoplethysmographic pickup of the blood flow pulse is artifactual noise produced by motion of the photo-sensitive cell and light source with respect to the pickup site. If the site is illuminated by broad spectrum "white" light, all of this light will be reflected from the structures within the tissue and as these structures move with respect to the pickup, noise will be generated which adversely affects the accuracy of the measurement.